3d flash memory cells which discourage cross-cell electrical tunneling

ABSTRACT

3-d flash memory cells and methods of manufacture are described. The devices and methods recess a compound floating gate in between the silicon oxide slabs which reduces the quantum probability of electron tunneling between vertically adjacent storage cells. The devices and methods further include a high work function nanocrystalline metal in the compound floating gate. A polysilicon buffer layer forms a portion of the compound floating gate. The polysilicon buffer layer allows the high work function nanocrystalline metal to be selectively deposited. The polysilicon buffer layer further protects the high work function nanocrystalline metal from oxidation with the gate oxide subsequently formed on the other side.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the benefit of U.S. Prov. Pat. App. No. 62/524,915 filed Jun. 26, 2017, and titled “3D FLASH MEMORY CELLS WHICH DISCOURAGE CROSS-CELL ELECTRICAL TUNNELING” by Purayath et al. The disclosure of 62/524,915 is hereby incorporated by reference in its entirety for all purposes.

FIELD

Embodiments of the invention relate to 3-d flash memory cells and methods of forming 3-d flash memory.

BACKGROUND

Integrated circuits are made possible by processes which produce intricately patterned material layers on substrate surfaces. Producing patterned material on a substrate requires controlled methods for removal of exposed material. Chemical etching is used for a variety of purposes including transferring a photoresist pattern into underlying layers, thinning layers or thinning lateral dimensions of features already present on the surface. Often it is desirable to have an etch process which etches one material faster than another helping e.g. a pattern transfer process proceed. Such an etch process is said to be selective of the first material. As a result of the diversity of materials, circuits and processes, etch processes have been developed that selectively remove one or more of a broad range of materials.

Dry etch processes are increasingly desirable for selectively removing material from semiconductor substrates. The desirability stems from the ability to gently remove material from miniature structures with minimal physical disturbance. Dry etch processes also allow the etch rate to be abruptly stopped by removing the gas phase reagents. Extremely selective etches have been developed recently to etch silicon nitride, silicon oxide or silicon while retaining the other materials.

A high density VNAND (or 3d-NAND) structure involves many storage layers arranged vertically. Introducing selective etch processes of silicon nitride and silicon oxide into 3d-NAND process flows may enable a greater number of storage layers to be included which may increase the storage density of completed devices. Methods are needed to broaden the utility of selective dry isotropic etch processes in vertical storage devices.

SUMMARY

3-d flash memory cells and methods of manufacture are described. The devices and methods recess a compound floating gate in between the silicon oxide slabs which reduces the quantum probability of electron tunneling between vertically adjacent storage cells. The devices and methods further include a high work function nanocrystalline metal in the compound floating gate which further discourages electron tunneling. A polysilicon buffer layer forms a portion of the compound floating gate. The polysilicon buffer layer allows the high work function nanocrystalline metal to be selectively deposited. The polysilicon buffer layer further protects the high work function nanocrystalline metal from oxidation with the gate oxide subsequently formed on the other side.

Exemplary devices may include 3-d flash memory cells. The 3-d flash memory cells include a stack of silicon oxide slabs. A tungsten slab selected from a plurality of tungsten slabs is disposed between each neighboring pair of vertically neighboring silicon oxide slabs in a memory hole of the 3-d flash memory cell. The plurality of tungsten slabs are recessed relative to the silicon oxide slabs away from the memory hole to form a plurality of recesses. The 3-d flash memory cells further include a plurality of compound floating gates each including a polysilicon buffer portion and a high work function metal portion. The plurality of compound floating gates are disposed entirely within the plurality of recesses. The stack of silicon oxide slabs extends beyond the plurality of compound floating gates into the memory hole. The 3-d flash memory cells further include a high-k dielectric liner disposed between the stack of silicon oxide slabs and the plurality of tungsten slabs. The high-k dielectric liner is further disposed between the plurality of compound floating gates and the tungsten slabs.

In some embodiments, the stack of silicon oxide slabs may be vertical. The high work function metal portion may include at least one of platinum, palladium, gold, iridium or ruthenium. The high work function metal portion may include nanocrystals of 30 Å or less. The stack of silicon oxide slabs may include at least fifteen silicon oxide slabs. One or more of the silicon oxide slabs may be less than 40 nm thick.

The present technology may also encompass methods. The methods may include forming a stack of alternating silicon oxide slabs and silicon nitride slabs. The methods further include forming a memory hole through the stack. The methods further include recessing the silicon nitride slabs from the silicon oxide slabs to form recesses. The methods further include forming polysilicon buffers in the recesses. The methods further include selectively forming high work function metal portions on the polysilicon buffers. The methods further include forming a conformal silicon nitride layer on the silicon oxide slabs and the high work function metal portions. The methods further include forming a conformal silicon oxide layer on the conformal silicon nitride layer. The methods further include forming a conformal polysilicon channel layer on the conformal silicon oxide layer. The methods further include filling the memory hole with dielectric. The methods further include removing the silicon nitride slabs. The methods further include forming a conformal high-k dielectric layer on the polysilicon buffers and the silicon oxide slabs. The methods further include forming a conformal titanium nitride layer on the conformal high-k dielectric layer. The methods further include forming tungsten on the conformal titanium nitride layer. Forming the conformal silicon nitride layer may include forming the conformal silicon nitride layer by atomic layer deposition. The thermionic work function of the high work function metal portions may be greater than 4.0 eV.

The present technology may also encompass additional 3-d flash memory cells. The 3-d flash memory cells include a stack of alternating silicon oxide slabs and tungsten slabs. The tungsten slabs are recessed compared to the silicon oxide slabs to form a plurality of recesses between vertically neighboring silicon oxide slabs. The 3-d flash memory cells further include a floating gate disposed entirely within at least one of the plurality of recesses. The floating gate includes a polysilicon buffer and a high work function metal. The polysilicon buffer is horizontally disposed between the high work function metal and one of the tungsten slabs. Neither the polysilicon buffer nor the high work function metal extends horizontally outside the plurality of recesses. The 3-d flash memory cells further include a high-k dielectric liner located between the polysilicon buffer and the corresponding tungsten slab (on the same horizontal level). The high-k dielectric liner also extends between and is located between the silicon oxide slabs and the tungsten slab. The plurality of the silicon oxide slabs may include at least fifteen silicon oxide slabs. The stack may be vertical. The 3-d flash memory cells may further include a plurality of floating gates each disposed between each neighboring pair of the silicon oxide slabs. The high work function metal may include a nanocrystalline high work function metal. Each of the silicon oxide slabs may be less than 50 nm thick. A thermionic work function of the high work function metal may be greater than 5.0 eV.

Additional embodiments and features are set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the specification or may be learned by the practice of the disclosed embodiments. The features and advantages of the disclosed embodiments may be realized and attained by means of the instrumentalities, combinations, and methods described in the specification.

DESCRIPTION OF THE DRAWINGS

A further understanding of the nature and advantages of the embodiments may be realized by reference to the remaining portions of the specification and the drawings.

FIG. 1A shows a schematic cross-sectional view of a substrate processing chamber according to embodiments.

FIG. 1B shows a schematic cross-sectional view of a portion of a substrate processing chamber according to embodiments.

FIG. 1C shows a bottom plan view of a showerhead according to embodiments.

FIG. 2 shows a top plan view of an exemplary substrate processing system according to embodiments.

FIG. 3A is a cross-sectional view of a patterned substrate during formation of a 3-d flash memory according to the present technology.

FIG. 3B is a cross-sectional view of a patterned substrate during formation of a 3-d flash memory according to the present technology.

FIG. 3C is a cross-sectional view of a patterned substrate during formation of a 3-d flash memory according to the present technology.

FIG. 3D is a cross-sectional view of a patterned substrate during formation of a 3-d flash memory according to the present technology.

FIG. 3E is a cross-sectional view of a patterned substrate during formation of a 3-d flash memory according to the present technology.

FIG. 3F is a cross-sectional view of a patterned substrate during formation of a 3-d flash memory according to the present technology.

FIG. 3G is a cross-sectional view of a patterned substrate during formation of a 3-d flash memory according to the present technology.

FIG. 4 is a flow chart of process for forming 3-d flash memory according to the present technology.

Several of the figures are included as schematics. It is to be understood that the figures are for illustrative purposes, and are not to be considered of scale unless specifically stated to be of scale. Additionally, as schematics, the figures are provided to aid comprehension and may not include all aspects or information compared to realistic representations, and may include superfluous or exaggerated material for illustrative purposes.

In the appended figures, similar components and/or features may have the same reference label. Further, various components of the same type may be distinguished by following the reference label by a letter that distinguishes among the similar components. If only the first reference label is used in the specification, the description is applicable to any one of the similar components having the same first reference label irrespective of the letter.

DETAILED DESCRIPTION

3-d flash memory cells and methods of manufacture are described. The devices and methods recess a compound floating gate in between the silicon oxide slabs which reduces the quantum probability of electron tunneling between vertically adjacent storage cells. The devices and methods further include a high work function nanocrystalline metal in the compound floating gate. A polysilicon buffer layer forms a portion of the compound floating gate. The polysilicon buffer layer allows the high work function nanocrystalline metal to be selectively deposited. The polysilicon buffer layer further protects the high work function nanocrystalline metal from oxidation with the gate oxide subsequently formed on the other side.

3-d flash memory (e.g. VNAND) has entered production recently beginning with a relatively small number of layers. An oxide-nitride-oxide (ONO) layer is conventionally deposited on the interior of a memory hole and charge is stored on the silicon nitride (N) layer. Multiple locations on the silicon nitride layer store independent collections of electrons to store the information in the VNAND device. The dielectric nature of the silicon nitride is relied on to avoid migration of electrons amongst neighboring locations. The present technology discourages migration of electrons by inlaying selectively deposited electron storage slabs between silicon oxide slabs and making the electron storage slabs from high work function metals.

Storage capacity increases proportional to the number of layers. Beyond about fifty layers the aspect ratio poses processing challenges. One approach to continue increasing the number of layers is to thin the silicon oxide slabs in the vertical dimension. Thinning the tungsten slabs is undesirable because of the resulting increase in resistance. However, thinning the silicon oxide slabs increases the quantum probability of electrons hopping vertically between neighboring storage cells. The devices and processes described herein have been determined to reduce the electron tunneling probability between neighboring cells despite a potentially strong local electric field promoting the tunneling. “Top” and “Up” will be used herein to describe portions/directions perpendicularly distal from the substrate plane and further away from the center of mass of the substrate in the perpendicular direction. “Vertical” will be used to describe items aligned in the “Up” direction towards the “Top”. Other similar terms may be used whose meanings will now be clear. The vertical memory hole may be circular as viewed from above. A further benefit of the devices and methods described herein include enabling further miniaturization in the vertical direction.

The approaches described herein are enabled, in part, by recently developed gas-phase etchants. Recently-developed gas-phase remote etch processes have been designed, in part, to remove the need to expose delicate surface patterns to liquid etchants. Liquid etchants are increasingly responsible for collapsing delicate surface patterns as linewidths are reduced. Liquid etchants also possess surface tension which make etchant penetration into the restricted spaces of memory holes difficult.

Although the remaining disclosure will routinely identify specific etching processes utilizing the disclosed technology, it will be readily understood that the systems and methods are equally applicable to deposition and cleaning processes as may occur in the described chambers and also include plasma or other reactive materials. Accordingly, the technology should not be considered to be so limited as for use with etching processes alone.

Hardware and processes may be selected to achieve the high etch selectivities which enable the processes and devices described herein. Exemplary hardware processes will be presented prior to introducing the processes and devices of the present technology. In embodiments, an ion suppressor (which may be the showerhead) may be used to provide radical and/or neutral species for gas-phase etching. The ion suppressor may also be referred to as an ion suppression element. In embodiments, for example, the ion suppressor is used to filter etching plasma effluents (including radical-fluorine) en route from the remote plasma region to the substrate processing region. The ion suppressor may be used to provide a reactive gas having a higher concentration of radicals than ions. Plasma effluents pass through the ion suppressor disposed between the remote plasma region and the substrate processing region. The ion suppressor functions to dramatically reduce or substantially eliminate ionic species traveling from the plasma generation region to the substrate. The ion suppressors described herein are simply one way to achieve a low electron temperature in the substrate processing region during the gas-phase etch processes described herein.

In embodiments, an electron beam is passed through the substrate processing region in a plane parallel to the substrate to reduce the electron temperature of the plasma effluents. A simpler showerhead may be used if an electron beam is applied in this manner. The electron beam may be passed as a laminar sheet disposed above the substrate in embodiments. The electron beam provides a source of neutralizing negative charge and provides a more active means for reducing the flow of positively charged ions towards the substrate and increasing the selectivity of silicon nitride in embodiments. The flow of plasma effluents and various parameters governing the operation of the electron beam may be adjusted to lower the electron temperature measured in the substrate processing region.

The electron temperature may be measured using a Langmuir probe in the substrate processing region during excitation of a plasma in the remote plasma. In embodiments, the electron temperature may be less than 0.5 eV, less than 0.45 eV, less than 0.4 eV, or less than 0.35 eV. These extremely low values for the electron temperature are enabled by the presence of the electron beam, showerhead and/or the ion suppressor. Uncharged neutral and radical species may pass through the electron beam and/or the openings in the ion suppressor to react at the substrate. Such a process using radicals and other neutral species can reduce plasma damage compared to conventional plasma etch processes that include sputtering and bombardment. Embodiments of the present invention are also advantageous over conventional wet etch processes where surface tension of liquids can cause bending and peeling of small features.

The substrate processing region may be described herein as “plasma-free” during the etch processes described herein. “Plasma-free” does not necessarily mean the region is devoid of plasma. Ionized species and free electrons created within the plasma region may travel through pores (apertures) in the partition (showerhead) at exceedingly small concentrations. The borders of the plasma in the chamber plasma region are hard to define and may encroach upon the substrate processing region through the apertures in the showerhead. Furthermore, a low intensity plasma may be created in the substrate processing region without eliminating desirable features of the etch processes described herein. All causes for a plasma having much lower intensity ion density than the chamber plasma region during the creation of the excited plasma effluents do not deviate from the scope of “plasma-free” as used herein.

FIG. 1A shows a cross-sectional view of an exemplary substrate processing chamber 101 with partitioned plasma generation regions within the processing chamber. During film etching, e.g., silicon oxide or silicon nitride, etc., a process gas may be flowed into chamber plasma region 115 through a gas inlet assembly 105. A remote plasma system (RPS) 102 may optionally be included in the system, and may process a first gas which then travels through gas inlet assembly 105. The inlet assembly 105 may include two or more distinct gas supply channels where the second channel (not shown) may bypass the RPS 102, if included. Accordingly, in embodiments the precursor gases may be delivered to the processing chamber in an unexcited state. The process gas may be excited within the RPS 102 prior to entering the chamber plasma region 115. Accordingly, the fluorine-containing precursor as discussed above, for example, may pass through RPS 102 or bypass the RPS unit in embodiments. Various other examples encompassed by this arrangement will be similarly understood.

A cooling plate 103, faceplate 117, ion suppressor 123, showerhead 125, and a substrate support 165 (also known as a pedestal), having a substrate 155 disposed thereon, are shown and may each be included according to embodiments. The pedestal 165 may have a heat exchange channel through which a heat exchange fluid flows to control the temperature of the substrate. This configuration may allow the substrate 155 temperature to be cooled or heated to maintain relatively low temperatures, such as between −20° C. to 200° C. The wafer support platter of the pedestal 165 may also be resistively heated to relatively high temperatures, such as from below 100° C. to above 1100° C.

In embodiments, the faceplate 117 may be flat (as shown) and include a plurality of through-channels used to distribute process gases. Plasma generating gases and/or plasma excited species, depending on use of the RPS 102, may pass through a plurality of holes, shown in FIG. 1B, in faceplate 117 for a more uniform delivery into the chamber plasma region 115.

Exemplary configurations may include having the gas inlet assembly 105 open into a gas supply region 158 partitioned from the chamber plasma region 115 by faceplate 117 so that the gases/species flow through the holes in the faceplate 117 into the chamber plasma region 115. Structural and operational features may be selected to prevent significant backflow of plasma from the chamber plasma region 115 back into the supply region 158, gas inlet assembly 105, and fluid supply system 110. The structural features may include the selection of dimensions and cross-sectional geometries of the apertures in faceplate 117 to deactivate back-streaming plasma. The operational features may include maintaining a pressure difference between the gas supply region 158 and chamber plasma region 115 that maintains a unidirectional flow of plasma through the showerhead 125. The faceplate 117, or a conductive top portion of the chamber, and showerhead 125 are shown with an insulating ring 120 located between the features, which allows an AC potential to be applied to the faceplate 117 relative to showerhead 125 and/or ion suppressor 123. The insulating ring 120 may be positioned between the faceplate 117 and the showerhead 125 and/or ion suppressor 123 enabling a capacitively coupled plasma (CCP) to be formed in the first plasma region. A baffle (not shown) may additionally be located in the chamber plasma region 115, or otherwise coupled with gas inlet assembly 105, to affect the flow of fluid into the region through gas inlet assembly 105.

The ion suppressor 123 may comprise a plate or other geometry that defines a plurality of apertures throughout the structure that are configured to suppress the migration of ionically-charged species out of chamber plasma region 115 while allowing uncharged neutral or radical species to pass through the ion suppressor 123 into an activated gas delivery region between the suppressor and the showerhead. In embodiments, the ion suppressor 123 may comprise a perforated plate with a variety of aperture configurations. These uncharged species may include highly reactive species that are transported with less reactive carrier gas through the apertures. As noted above, the migration of ionic species through the holes may be reduced, and in some instances completely suppressed. Controlling the amount of ionic species passing through the ion suppressor 123 may provide increased control over the gas mixture brought into contact with the underlying wafer substrate, which in turn may increase control of the deposition and/or etch characteristics of the gas mixture. For example, adjustments in the ion concentration of the gas mixture can significantly alter its etch selectivity, e.g., SiO:Si etch ratios, SiN:Si etch ratios, etc.

The plurality of holes in the ion suppressor 123 may be configured to control the passage of the activated gas, i.e., the ionic, radical, and/or neutral species, through the ion suppressor 123. For example, the aspect ratio of the holes, or the hole diameter to length, and/or the geometry of the holes may be controlled so that the flow of ionically-charged species in the activated gas passing through the ion suppressor 123 is reduced. The holes in the ion suppressor 123 may include a tapered portion that faces chamber plasma region 115, and a cylindrical portion that faces the showerhead 125. The cylindrical portion may be shaped and dimensioned to control the flow of ionic species passing to the showerhead 125. An adjustable electrical bias may also be applied to the ion suppressor 123 as an additional means to control the flow of ionic species through the suppressor.

The ion suppression element 123 may function to reduce or eliminate the amount of ionically charged species traveling from the plasma generation region to the substrate. Uncharged neutral and radical species may still pass through the openings in the ion suppressor to react with the substrate.

Showerhead 125 in combination with ion suppressor 123 may allow a plasma present in chamber plasma region 115 to avoid directly exciting gases in substrate processing region 133, while still allowing excited species to travel from chamber plasma region 115 into substrate processing region 133. In this way, the chamber may be configured to prevent the plasma from contacting a substrate 155 being etched. This may advantageously protect a variety of intricate structures and films patterned on the substrate, which may be damaged, dislocated, or otherwise warped if directly contacted by a generated plasma. Additionally, when plasma is allowed to contact the substrate or approach the substrate level, the etch selectivity may decrease.

The processing system may further include a power supply 140 electrically coupled with the processing chamber to provide electric power to the faceplate 117, ion suppressor 123, showerhead 125, and/or pedestal 165 to generate a plasma in the chamber plasma region 115 or processing region 133. The power supply may be configured to deliver an adjustable amount of power to the chamber depending on the process performed. Such a configuration may allow for a tunable plasma to be used in the processes being performed. Unlike a remote plasma unit, which is often presented with on or off functionality, a tunable plasma may be configured to deliver a specific amount of power to chamber plasma region 115. This in turn may allow development of particular plasma characteristics such that precursors may be dissociated in specific ways to enhance the etching profiles produced by these precursors.

A plasma may be ignited either in chamber plasma region 115 above showerhead 125 or substrate processing region 133 below showerhead 125 in processes where “plasma-free” is not necessary. A plasma may be present in chamber plasma region 115 to produce the radical-fluorine precursors from an inflow of the fluorine-containing precursor. An AC voltage typically in the radio frequency (RF) range may be applied between the conductive top portion of the processing chamber, such as faceplate 117, and showerhead 125 and/or ion suppressor 123 to ignite a plasma in chamber plasma region 115 during deposition. An RF power supply may generate a high RF frequency of 13.56 MHz but may also generate other frequencies alone or in combination with the 13.56 MHz frequency.

Plasma power can be of a variety of frequencies or a combination of multiple frequencies. In the exemplary processing system the plasma may be provided by RF power delivered to faceplate 117 relative to ion suppressor 123 and/or showerhead 125. The RF power may be between 10 watts and 5000 watts, between 100 watts and 2000 watts, between 200 watts and 1500 watts, or between 200 watts and 1000 watts in embodiments. The RF frequency applied in the exemplary processing system may be low RF frequencies less than 200 kHz, high RF frequencies between 10 MHz and 15 MHz, or microwave frequencies greater than 1 GHz in embodiments. The plasma power may be capacitively-coupled (CCP) or inductively-coupled (ICP) into the remote plasma region.

Excited effluents including radical fluorine formed from a fluorine-containing precursor, may be flowed into the processing region 133 by embodiments of the showerhead described herein. Excited species derived from the process gas in chamber plasma region 115 may travel through apertures in the ion suppressor 123, and/or showerhead 125 and react with an additional precursor flowing into the processing region 133 from a separate portion of the showerhead. Alternatively, if all precursor species are being excited in chamber plasma region 115, no additional precursors may be flowed through the separate portion of the showerhead. Little or no plasma may be present in the processing region 133 during the remote plasma etch process in embodiments. Excited derivatives of the precursors may combine in the region above the substrate and/or on the substrate to etch structures or remove species from the substrate.

Some dry-etch processes involve the exposure of a substrate to remote plasma by-products formed from one or more precursors. Secondary precursors may be introduced directly into the substrate processing region without passing through the remote plasma. Selection of precursors results in an etch process which is selective of a specific material. For example, remote plasma generation of a fluorine precursor in combination with an oxygen precursor results in a selective etch of silicon nitride. In contrast, a remote plasma generation of a fluorine precursor and concurrent introduction of moisture directly into the substrate processing region results in a selective etch of silicon oxide at about room temperature. Doped silicon oxide may be selectively removed by combining radical-fluorine with moisture (by-passing any plasma excitation) and maintaining the substrate temperature at about 60° C. rather than near room temperature. Specific examples of precursors will now be presented. In embodiments intended to preferentially etch doped or undoped silicon oxide, the fluorine-containing precursor may be nitrogen trifluoride and the secondary precursor (excited only by the radical-fluorine) may be water vapor (H₂O). Water vapor, when used, may be delivered using a mass flow meter (MFM), an injection valve, or by commercially available water vapor generators. Gaseous precursors may be delivered using mass flow controllers (MFC's). In embodiments intended to preferentially etch silicon nitride, the fluorine-containing precursor may be nitrogen trifluoride which may be combined with a second precursor and the mixture may be excited in a remote plasma. The second precursor in the mixture may be oxygen (O₂) or another oxygen-containing precursor. In embodiments intended to preferentially etch silicon (e.g. polysilicon), the fluorine-containing precursor may be nitrogen trifluoride which may be combined with a second precursor and the mixture may be excited in a remote plasma. The second precursor in the mixture may be hydrogen (H₂). Other combinations of precursors have been developed for each of the selective etches used herein, however, the combinations described are sufficient to enable the process flows.

FIG. 1B shows a detailed view of the features affecting the processing gas distribution through faceplate 117. As shown in FIG. 1A and FIG. 1B, faceplate 117, cooling plate 103, and gas inlet assembly 105 intersect to define a gas supply region 158 into which process gases may be delivered from gas inlet 105. The gases may fill the gas supply region 158 and flow to the chamber plasma region 115 through apertures 159 in faceplate 117. The apertures 159 may be configured to direct flow in a substantially unidirectional manner such that process gases may flow into processing region 133, but may be partially or fully prevented from backflow into the gas supply region 158 after traversing the faceplate 117.

The gas distribution assemblies such as showerhead 125 for use in the processing chamber section 101 may be referred to as dual channel showerheads (DCSH) and are additionally detailed in the embodiments described in FIG. 1A as well as FIG. 1C herein. The dual channel showerhead may provide for etching processes that allow for separation of etchants outside of the processing region 133 to provide limited interaction with chamber components and each other prior to being delivered into the processing region.

The showerhead 125 may comprise an upper plate 114 and a lower plate 116. The plates may be coupled with one another to define a volume 118 between the plates. The coupling of the plates may be so as to provide first fluid channels 119 through the upper and lower plates, and second fluid channels 121 through the lower plate 116. The formed channels may be configured to provide fluid access from the volume 118 through the lower plate 116 via second fluid channels 121 alone, and the first fluid channels 119 may be fluidly isolated from the volume 118 between the plates and the second fluid channels 121. The volume 118 may be fluidly accessible through a side of the gas distribution assembly 125. Although the exemplary system of FIGS. 1A-1C includes a dual-channel showerhead, it is understood that alternative distribution assemblies may be utilized that maintain first and second precursors fluidly isolated prior to the processing region 133. For example, a perforated plate and tubes underneath the plate may be utilized, although other configurations may operate with reduced efficiency or not provide as uniform processing as the dual-channel showerhead as described.

In the embodiment shown, showerhead 125 may distribute via first fluid channels 119 process gases which contain plasma effluents upon excitation by a plasma in chamber plasma region 115. In embodiments, the process gas introduced into the RPS 102 and/or chamber plasma region 115 may contain fluorine, e.g., CF₄, NF₃ or XeF₂. The process gas may also include a carrier gas such as helium, argon, nitrogen (N₂), etc. Plasma effluents may include ionized or neutral derivatives of the process gas and may also be referred to herein as a radical-fluorine precursor referring to the atomic constituent of the process gas introduced.

FIG. 1C is a bottom view of a showerhead 125 for use with a processing chamber in embodiments. Showerhead 125 corresponds with the showerhead shown in FIG. 1A. Through-holes 131, which show a view of first fluid channels 119, may have a plurality of shapes and configurations to control and affect the flow of precursors through the showerhead 125. Small holes 127, which show a view of second fluid channels 121, may be distributed substantially evenly over the surface of the showerhead, even amongst the through-holes 131, which may help to provide more even mixing of the precursors as they exit the showerhead than other configurations.

The chamber plasma region 115 or a region in an RPS may be referred to as a remote plasma region. In embodiments, the radical precursor, e.g., a radical-fluorine precursor, is created in the remote plasma region and travels into the substrate processing region where it may or may not combine with additional precursors. In embodiments, the additional precursors are excited only by the radical-fluorine precursor. Plasma power may essentially be applied only to the remote plasma region in embodiments to ensure that the radical-fluorine precursor provides the dominant excitation.

Combined flow rates of precursors into the chamber may account for 0.05% to 20% by volume of the overall gas mixture; the remainder being carrier gases. The fluorine-containing precursor may be flowed into the remote plasma region, but the plasma effluents may have the same volumetric flow ratio in embodiments. In the case of the fluorine-containing precursor, a purge or carrier gas may be first initiated into the remote plasma region before the fluorine-containing gas to stabilize the pressure within the remote plasma region. Substrate processing region 133 can be maintained at a variety of pressures during the flow of precursors, any carrier gases, and plasma effluents into substrate processing region 133. The pressure may be maintained between 0.1 mTorr and 100 Torr, between 1 Torr and 20 Torr or between 1 Torr and 5 Torr in embodiments.

Embodiments of the deposition systems may be incorporated into larger fabrication systems for producing integrated circuit chips. FIG. 2 shows one such processing system (mainframe) 201 of deposition, etching, baking, and curing chambers in embodiments. In the figure, a pair of front opening unified pods (load lock chambers 202) supply substrates of a variety of sizes that are received by robotic arms 204 and placed into a low pressure holding area 206 before being placed into one of the substrate processing chambers 208 a-f. A second robotic arm 210 may be used to transport the substrate wafers from the holding area 206 to the substrate processing chambers 208 a-f and back. Each substrate processing chamber 208 a-f, can be outfitted to perform a number of substrate processing operations including the dry etch processes described herein in addition to cyclical layer deposition (CLD), atomic layer deposition (ALD), chemical vapor deposition (CVD), physical vapor deposition (PVD), etch, pre-clean, degas, orientation, and other substrate processes.

The substrate processing chambers 208 a-f may be configured for depositing, annealing, curing and/or etching a film on the substrate wafer. In one configuration, chambers 208 a-b, may be configured to etch silicon nitride, chambers 208 c-d may be configured to etch silicon oxide, and chambers 208 e-f may be configured to etch silicon.

Reference is now made to FIGS. 3A-3G concurrently with references to FIG. 4. A method 2010 of forming a 3-d flash memory cell may involve forming a stack of alternating silicon oxide slabs 1120 a and silicon nitride slabs 1070 a (FIG. 3A) on a substrate 1010 a. The method further includes forming a memory hole through the stack in operation 2100. The silicon oxide slabs 1120 b and silicon nitride slabs 1070 b are shown in FIG. 3B after formation of the memory hole 1100. The substrate 1010 a may be etched during a reactive ion etch process (used to form the memory hole 1100) to form impressions in the substrate 1010 b.

The method may include selectively etching the silicon nitride slabs to recess the silicon nitride slabs 1070 c in operation in operation 2200. A compound floating gate is then placed inside the recess in silicon nitride slabs 1070 c. The compound floating gate is formed by first forming a polysilicon buffer 1300 inside the recess in operation 2300. The polysilicon deposition may form polysilicon outside the recess and the polysilicon may be separated and/or recessed to form the polysilicon buffer 1300 shown in FIG. 3C. The stack of alternating layers (and the memory holes) may include more than 30 pairs of slabs, more than 35 pairs of slabs, more than 40 pairs of slabs, more than 45 pairs of slabs, or more than 50 pairs of slabs in embodiments. The stack of alternating layers (and the memory holes) may include between 30 and 90 pairs of slabs, between 35 and 75 pairs of slabs or between 40 and 60 pairs of slabs according to embodiments. The memory hole may have a width of between 350 Å and 3000 Å, between 500 Å and 2000 Å or between 700 Å and 1300 Å according to embodiments. The memory hole may be circular as viewed from above in which case the width is the diameter. The memory hole may have an aspect ratio (height to width) of between 15:1 and 50:1, between 20:1 and 40:1 or between 25:1 and 35:1 in embodiments. The figures will show a smaller number of pairs to simplify the drawings but the possible presence of additional layers will be indicated using the repetition extension line 1150.

A high work function metal 1350 is selectively formed on the polysilicon buffer 1300 in operation 2400 and the resulting structure is shown in FIG. 3C. The high work function metal 1350 has been found to desirably avoid deposition on silicon oxide while depositing readily on polysilicon. In use, charge will be stored predominantly on the high work function metal 1350. The terms “floating gate” and “compound floating gate” each refer to both the polysilicon buffer 1300 and the high work function metal 1350. Recessing the high work function metal 1350 mostly or entirely within the recess, in embodiments, results in the benefit of the reduction in a deleterious loss of stored charge from one cell to a vertically neighboring cell. The high work function metal 1350 may be flush with the edge of the silicon oxide slabs 1120 b as shown on the left side of FIG. 3C, or may be inset from the silicon oxide slabs 1120 b in embodiments. The alternative inset arrangement is shown in the inset on the right side of FIG. 3C. When an inset is present, the width or diameter of a memory hole is measured between the high work function metal portions rather than between the silicon oxide slabs. The silicon oxide slabs 1120 b may protrude further into the memory hole 1100 than the high work function metal 1350 according to embodiments. One or all of a vertical sequence of high work function metal 1350 may be disposed entirely within one or a vertical sequence of recesses in embodiments.

A thin conformal layer of silicon nitride 1410 may be formed on the silicon oxide slabs 1120 b and also on the high work function metal 1350 in operation 2500. The thin conformal layer of silicon nitride 1410 may be nearly vertical which correlates with an improvement in quality of the thin conformal layer of silicon nitride 1410 as well as in subsequent conformal layers. A conformal layer of silicon oxide 1420 is then formed (operation 2600) on the thin conformal layer of silicon nitride 1410. The conformal layer of silicon oxide 1420 will serve the function of the gate oxide for the 3d flash memory device. The integrity of the conformal layer of silicon oxide 1420 is improved for vertical (straight) layers which is facilitated by previously forming the high work function metal 1350 to be close to flush with the silicon oxide slabs 1120 b without going “beyond” flush according to embodiments. The silicon oxide slabs 1120 b may extend into the memory hole more than the high work function metal 1350 by no more than 10 Å, by no more than 7.5 Å or by no more than 5 Å in embodiments. A polysilicon channel layer 1430 is formed on the conformal layer of silicon oxide 1420 (also in operation 2600). The polysilicon channel layer 1430 is conformal according to embodiments. Dielectric gapfill 1440 may be used to fill the remaining gaps left following the formation of polysilicon channel layer 1430 before proceeding to operation 2700 in embodiments. The device is shown in FIG. 3D prior to operation 2700.

The thickness of the polysilicon channel layer 1430 may be between 3 nm and 15 nm, between 4 nm and 13 nm or between 5 nm and 10 nm in embodiments. The thickness of the conformal layer of silicon oxide 1420 may be between 1 nm and 12 nm, between 2 nm and 9 nm or between 3 nm and 6 nm according to embodiments. The thickness of the thin conformal layer of silicon nitride 1410 may be between 1 nm and 12 nm, between 2 nm and 9 nm or between 3 nm and 6 nm in embodiments.

The silicon nitride slabs 1070 c are removed in operation 2700. The resulting structure is shown in FIG. 3E. A conformal high-k layer is formed on the polysilicon buffer 1300 and the exposed portions of the silicon oxide slabs 1120 b. A conformal barrier layer of titanium nitride is then formed on the conformal high-k layer in operation 2800. Collectively, the conformal high-k layer and the conformal barrier layer are shown in FIG. 3F as compound barrier layer 1450. Tungsten slabs 1460 are deposited on the compound barrier layer 1450 as shown in FIG. 3G (operation 2900). Tungsten slabs 1460 may optionally be separated using an etch process if tungsten is undesirably deposited in a way that shorts the tungsten slabs 1460 together electrically.

The high work function metal may include one of platinum, palladium, gold, iridium or ruthenium in embodiments. The high work function metal may consist of platinum, palladium, gold, iridium and/or ruthenium according to embodiments. The thermionic work function of the high work function metal may be greater than 4.0 eV, greater than 4.5 eV, greater than 5.0 eV, greater than 5.5 eV or greater than 6.0 eV in embodiments. The high work function metal portion may comprise nanocrystals having an average diameter of 30 Å or less, 25 Å or less, 20 Å or less, or 15 Å or less, in embodiments. The conformal high-k dielectric (which may be defined as the first portion of the conformal barrier layer) may be aluminum oxide or hafnium oxide according to embodiments. The conformal high-k dielectric may be a tri-layer, band engineered to increase the tunneling efficiency of electrons onto the floating gate. The conformal high-k dielectric may be HfOx/AlO/HfOx or SiOx/AlO/SiOx in embodiments.

Following assembly and during use of the resulting integrated circuit, in embodiments, the following dimensions may characterize the various components of the device. The height (e.g. the thickness or the vertical dimension) of one or more of the silicon oxide slabs may be less than 50 nm, less than 45 nm, less than 40 nm, less than 35 nm or less than 30 nm according to embodiments. The present technology may enable higher density VNAND by reducing the thickness of the silicon oxide slabs which still maintain sufficient separation of adjacent storage cells to prevent quantum mechanical tunneling. The height (e.g. the thickness or the vertical dimension) of one or more (or all) of the high work function metal slabs may be less than 60 nm, less than 55 nm, less than 50 nm, less than 45 nm or less than 40 nm according to embodiments. The height of one or more (or all) of the high work function metal slabs may be between 20 nm and 40 nm, between 25 nm and 35 nm or between 27 nm and 33 nm in embodiments.

As used herein “substrate” may be a support substrate with or without layers formed thereon. The patterned substrate may be an insulator or a semiconductor of a variety of doping concentrations and profiles and may, for example, be a semiconductor substrate of the type used in the manufacture of integrated circuits. Exposed “silicon” or “polysilicon” of the patterned substrate is predominantly Si but may include minority concentrations of other elemental constituents such as nitrogen, oxygen, hydrogen and carbon. Exposed “silicon” or “polysilicon” may consist of or consist essentially of silicon. Exposed “silicon nitride” of the patterned substrate is predominantly Si₃N₄ but may include minority concentrations of other elemental constituents such as oxygen, hydrogen and carbon. Exposed “silicon nitride” may consist essentially of or consist of silicon and nitrogen. Exposed “silicon oxide” of the patterned substrate is predominantly SiO₂ but may include minority concentrations of other elemental constituents such as nitrogen, hydrogen and carbon. In embodiments, silicon oxide films etched using the methods taught herein consist essentially of or consist of silicon and oxygen.

The term “precursor” is used to refer to any process gas which takes part in a reaction to either remove material from or deposit material onto a surface. “Plasma effluents” describe gas exiting from the chamber plasma region and entering the substrate processing region. Plasma effluents are in an “excited state” wherein at least some of the gas molecules are in vibrationally-excited, dissociated and/or ionized states. A “radical precursor” is used to describe plasma effluents (a gas in an excited state which is exiting a plasma) which participate in a reaction to either remove material from or deposit material on a surface. “Radical-fluorine” are radical precursors which contain fluorine but may contain other elemental constituents. The phrase “inert gas” refers to any gas which does not form chemical bonds when etching or being incorporated into a film. Exemplary inert gases include noble gases but may include other gases so long as no chemical bonds are formed when (typically) trace amounts are trapped in a film.

The terms “gap” and “trench” are used throughout with no implication that the etched geometry has a large horizontal aspect ratio. Viewed from above the surface, trenches may appear circular, oval, polygonal, rectangular, or a variety of other shapes. A trench may be in the shape of a moat around an island of material. The term “memory hole” is used to refer to a low aspect ratio trench (as viewed from above) which may or may not be filled with material. As used herein, an isotropic or a conformal etch process refers to a generally uniform removal of material on a surface in the same shape as the surface, i.e., the surface of the etched layer and the pre-etch surface are generally parallel. A conformal layer, analogously, has top and bottom surfaces which are generally parallel to the deposition surface. A person having ordinary skill in the art will recognize that the etched interface likely cannot be 100% conformal and thus the term “generally” allows for acceptable tolerances.

In the preceding description, for the purposes of explanation, numerous details have been set forth in order to provide an understanding of various embodiments of the present technology. It will be apparent to one skilled in the art, however, that certain embodiments may be practiced without some of these details, or with additional details.

Having disclosed several embodiments, it will be recognized by those of skill in the art that various modifications, alternative constructions, and equivalents may be used without departing from the spirit of the embodiments. Additionally, a number of well-known processes and elements have not been described in order to avoid unnecessarily obscuring the present technology. Accordingly, the above description should not be taken as limiting the scope of the technology. Additionally, methods or processes may be described as sequential or in steps, but it is to be understood that the operations may be performed concurrently, or in different orders than listed.

Where a range of values is provided, it is understood that each intervening value, to the smallest fraction of the unit of the lower limit, unless the context clearly dictates otherwise, between the upper and lower limits of that range is also specifically disclosed. Any narrower range between any stated values or unstated intervening values in a stated range and any other stated or intervening value in that stated range is encompassed. The upper and lower limits of those smaller ranges may independently be included or excluded in the range, and each range where either, neither, or both limits are included in the smaller ranges is also encompassed within the technology, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included.

As used herein and in the appended claims, the singular forms “a”, “an”, and “the” include plural references unless the context clearly dictates otherwise. Thus, for example, reference to “a precursor” includes a plurality of such precursors, and reference to “the layer” includes reference to one or more layers and equivalents thereof known to those skilled in the art, and so forth.

Also, the words “comprise(s)”, “comprising”, “contain(s)”, “containing”, “include(s)”, and “including”, when used in this specification and in the following claims, are intended to specify the presence of stated features, integers, components, or operations, but they do not preclude the presence or addition of one or more other features, integers, components, operations, acts, or groups. 

1. A 3-d flash memory cell, the 3-d flash memory cell comprising: a stack of silicon oxide slabs, wherein a tungsten slab selected from a plurality of tungsten slabs is disposed between each neighboring pair of vertically neighboring silicon oxide slabs in a memory hole of the 3-d flash memory cell, wherein the plurality of tungsten slabs are recessed relative to the silicon oxide slabs away from the memory hole to form a plurality of recesses; a plurality of compound floating gates each comprising a polysilicon buffer portion and a high work function metal portion, wherein the plurality of compound floating gates are disposed entirely within the plurality of recesses, wherein the stack of silicon oxide slabs extends beyond the plurality of compound floating gates into the memory hole; and a high-k dielectric liner disposed between the stack of silicon oxide slabs and the plurality of tungsten slabs, wherein the high-k dielectric liner is further disposed between the plurality of compound floating gates and the tungsten slabs.
 2. The 3-d flash memory cell of claim 1 wherein the stack of silicon oxide slabs is vertical.
 3. The 3-d flash memory cell of claim 1 wherein the high work function metal portion comprises at least one of platinum, palladium, gold, iridium or ruthenium.
 4. The 3-d flash memory cell of claim 1 wherein the high work function metal portion comprises nanocrystals of 30 Å or less.
 5. The 3-d flash memory cell of claim 1 wherein the stack of silicon oxide slabs comprises at least fifteen silicon oxide slabs.
 6. The 3-d flash memory cell of claim 1 wherein one or more of the silicon oxide slabs is less than 40 nm thick.
 7. A method of forming a 3-d flash memory cell, the method comprising: forming a stack of alternating silicon oxide slabs and silicon nitride slabs; forming a memory hole through the stack; recessing the silicon nitride slabs from the silicon oxide slabs to form recesses; forming polysilicon buffers in the recesses; selectively forming high work function metal portions on the polysilicon buffers; forming a conformal silicon nitride layer on the silicon oxide slabs and the high work function metal portions; forming a conformal silicon oxide layer on the conformal silicon nitride layer; forming a conformal polysilicon channel layer on the conformal silicon oxide layer; filling the memory hole with dielectric; removing the silicon nitride slabs; forming a conformal high-k dielectric layer on the polysilicon buffers and the silicon oxide slabs; forming a conformal titanium nitride layer on the conformal high-k dielectric layer; and forming tungsten on the conformal titanium nitride layer.
 8. The method of claim 7 wherein forming the conformal silicon nitride layer comprises forming the conformal silicon nitride layer by atomic layer deposition.
 9. The method of claim 7 wherein a thermionic work function of the high work function metal portions is greater than 4.0 eV.
 10. A 3-d flash memory cell, the 3-d flash memory cell comprising: a stack of alternating silicon oxide slabs and tungsten slabs comprising a plurality of the silicon oxide slabs, wherein the tungsten slabs are recessed compared to the silicon oxide slabs to form a plurality of recesses between vertically neighboring silicon oxide slabs; a floating gate disposed entirely within at least one of the plurality of recesses, wherein the floating gate comprises a polysilicon buffer and a high work function metal and wherein the polysilicon buffer is horizontally disposed between the high work function metal and one of the tungsten slabs, wherein neither the polysilicon buffer nor the high work function metal extends horizontally outside the plurality of recesses; and a high-k dielectric liner disposed between the polysilicon buffer and one of the tungsten slabs, wherein the high-k dielectric liner is further disposed between the silicon oxide slabs and the tungsten slab.
 11. The 3-d flash memory cell of claim 10 wherein the plurality of the silicon oxide slabs comprises at least fifteen silicon oxide slabs.
 12. The 3-d flash memory cell of claim 10 wherein the stack is vertical.
 13. The 3-d flash memory cell of claim 10 further comprising a plurality of floating gates each disposed between each neighboring pair of the silicon oxide slabs.
 14. The 3-d flash memory cell of claim 10 wherein the high work function metal comprises a nanocrystalline high work function metal.
 15. The 3-d flash memory cell of claim 10 wherein each of the silicon oxide slabs is less than 50 nm thick.
 16. The 3-d flash memory cell of claim 10 wherein a thermionic work function of the high work function metal is greater than 5.0 eV. 